With ES you can get any rate you are eligible for. If senior fares are lower, and someone in the cabin is 55 or older, and your booking is ES, you can get the senior fare. If I think there is a good chance I won't go on a cruise, then I won't book ES, but if I'm certain I'm going on the cruise I think ES is best.

Correct..in fact I used it yesterday. I filed the online form yesterday morning as the Sr rate on B6 dropped below my ES fare. They approved it by 7:00 pm. It now shows as a Sr rate category so not sure it could be used again as it's no longer a ES.

I never book ES I book something more flexible, but by final pmt date if I'm 100% sure about cruise I will change it to ES just in case it does drop in future. (As it did on the Breeze BC6)